I use peat moss. But I've heard that, in its natural state, it's a breeding ground for parasites and other harmful organisms. Good for plants, not so good for snakes.

But here's what I do: I bake it in the oven until it reaches a temperature of 200F. That pretty much kills just about anything, and even dries it out some to make it more suitable. But it does stink of the house real good, so you have been warned.